This invention relates to novel metal coating which exhibit exceptional hardness. More particularly this invention relates to metal coatings containing nickel and boron and to the reductive deposition of said coatings on the surfaces of substrate articles from aqueous solutions at an alkaline pH.
The plating or deposition of metal alloys by chemical or electrochemical reduction of metal ions on the surface of an article to modify its surface characteristics for both decorative and functional purposes is well known in the art. Of particular commercial significance is the deposition of metal/metal alloy coatings on both metal and activated non-metal substrates to enhance surface hardness and resistance to corrosion and wear. Nickel-boron and cobalt-boron alloy coatings are recognized in the art for their hardness and associated wear-resistance. The patent literature reflects an ongoing research and development effort in the area of nickel-boron coatings with the goal of producing still harder, more corrosion resistance coatings. For example, see, U.S. Pat. Nos. 5,019,163; 3,738,849; 3,674,447; 3,378,400; 3,045,334. The art has recognized that when borohydride is used in a nickel/boron-plating bath a harder coating is achieved. However, borohydride, is very unstable in the bath. The solution to the stability problem has been to add stabilizers such as thallium sulfate or lead chloride. The addition of the stabilizers had the effect of interfering with the formation of the nickel coating thereby negatively impacting the hardness of the coating. Because the stabilizer is not co-deposited in the coating in accordance with the present invention the nickel coating is remarkably more hard than those described in the prior art.
To-date nickel/boron coatings have always included a stabilizer as a third element. The only exception is a dimethyl borane coating. This type of coating does not have the stabilizer present in the coating. This process has very limited application because the bath has a very slow deposit rate and the coating is very thin. The deposit rates are in the order of 0.00015 inches per hour. The thickness of the deposit is limited to about 0.0001-0.0002 inch. These deposits are too thin to be used for wear surfaces.
It is therefore a general object of this invention is to provide an article of manufacture coated on at least a portion of its surface with a hard, ductile, wear and corrosion resistant metal coating comprising both nickel and boron, without the co-deposition of a stabilizer.
Yet another object of this invention is to provide coating baths from which a hard, ductile, wear and corrosion resistant coating can be deposited on at least a portion of the surface of a metal or activated non-metal substrate.